3 days in Wales

I'd always vote for the north - Conwy, Caernarfon, Beaumaris, Llanberis, some of my favourite places. I'd recommend finding a guidebook and taking a flip through and see what tweaks your interests.

I've never been to Wales, so I have no specific suggestions. But, I do have very specific suggestions on how you should refine your question to get better help. In order for anyone to answer your question in a meaningful way, we need a LOT more information. 1. Where are you going before and after Wales? What is the itinerary for the rest of your trip? 2. What drew you to Wales? What do you hope to get out of the visit? 3. Will you have a car? (be aware that from other posts, I know that Wales' rail network is skeletal, and buses go to only some of the country). 4. Who is "we" (who is going with you - ages, interests, etc)? 5. When are you going?
6. What is your budget? Etc, etc. The more specifics you give, the better others can answer.

How many days???

^ Her subject says 3. :)

Ugh.

Do you like castles or little trains? There are so many wonderful ones in Wales. Also, the countryside is lovely and staying on B & B farms is a wonderful way to experience Wales.